The present invention relates to novel and improved planographic masters and a method for prepareing planographic printing plates suitable for dry planographic printing, using the planographic masters.
Planographic printing plates in the prior art are prepared by providing, on the surface of substrate, image areas or oleophilic areas which have an affinity with a printing ink but are hardly wetted with water and non-image areas or hydrophilic areas which have an ink-repellent property but are readily wetted with water. The printing process with the printing plates is carried out by first moistening the non-image areas with a dampening solution, giving the printing ink to the image areas and then transferring the printing ink retained on the image areas to sheets of paper of the like to be printed.
Various troublesome problems have been encountered with the prior art planographic printing plates as a consequence of the use of a dampening solution. Some of the problems are (1) that the printing ink is apt to become emulsified, (2) that the density of the printed colors is controlled with difficulty, (3) that the process tends to cause dimensional changes to printed sheets and bring about inferiority in precision printing or multi-colored printing, (4) that complicated structure and mechanisms are required by printing presses employed in the press, and (5) that the preparation of the printing plates is very complicated and requires very fine technique due to the etching process involved.
Recently, there have been proposed improved planographic masters with which no etching process is involved in the plate making process concerned and with which dry planographic printing can be performed without a dampening solution, taking the advantage of the excellent ink-repellency of a silicone or an organopolysiloxane composition as the material for forming the non-image areas of the printing plates. (See, for example, Japanese Patent Public Disclosure 48-19305 disclosing a planographic master which is prepared by coating the substrate with a substance capable of electrostatically forming latent images and providing, over the layer of coating then dried, a cured layer of a silicone.)
However, the above printing plates have been found unsatisfactory in view of their inferior printing durability due to lack of bonding strength between the image area-forming material and the silicone layer. This is so, for example, when the formation of ink-receptive image areas on the dry planographic printing plates is intended by providing an image area-forming material on the planographic masters by means of the electrophotographic process or the socalled direct image process involving typewriting or handwriting or drawing, both of which processes have recently gained wide-spread acceptance in printing, especially with light printing machines suitable for office use, where the desired printing durability is rather small, but sufficient such that printed copies as many as about 1,000 to 2,000 are obtained by a single printing plate.
Further, in order to improve the bonding strength between the image area-forming material and the silicone layer on the printing plates, various methods have been proposed (see Japanese Patent Public Disclosures Nos. 49-21204, 50-71405, 50-71406, 50-78403, 50-78404, 50-78405, and 51-16105). However, the proposed methods have been found defective with respect to ink-repellency, printing durability, storage stability, or the like as well as easiness in the plate making process.